This invention relates to processes and apparatus for applying to a surface of a support member at least one ribbon-like stream of a first coating composition adjacent to and in edge contact with at least one second ribbon-like stream of a second coating composition to form a unitary layer on the surface of the support member.
Numerous techniques have been devised to form on a substrate a coating of one composition side-by-side with another coating of a second composition. One of these techniques involves two separate passes of the substrate to permit application of the first coating followed by a second pass to allow application of the second coating. Unfortunately, multiple passes require more time, duplicate handling, and highly sophisticated equipment for alignment of the coatings. Further, where heating of the deposited coatings is necessary for curing or drying, the process may require two separate heating steps. Moreover, multiple passes increase the likelihood of damage to the substrate or coatings, particularly for coated substrates that demand precision tolerances such as flexible photoreceptors for high speed electrostatographic copying and duplicating machines. When multiple pass techniques are utilized to apply side-by-side coatings, it is often difficult to achieve uniform edge to edge contact between the coatings. Moreover, because of overlapping deposits, differences in physical properties including surface tension, and lateral movement of previously or subsequently deposited coatings, a bead frequently forms along the border of side-by-side coatings. This bead causes a ridge to form above the bead as well as in the substrate below the bead when the coated support member is a flexible web which is subsequently rolled for storage, shipment of further processing. This ridge is undesirable in precision machines and can cause adverse effects such as electrical arcing and coating damage due to contact with closely spaced machine components. Moreover, a thick bead at the boundary between side-by-side layers tends to promote the formation of blisters when the coatings are applied as solutions containing volatile solvents. In addition, where fluids are used which have a tendency to spread over each other, the bead acts as a reservoir to promote greater spreading of the fluids over each other.
In order to form side-by-side coatings or webs in a single pass, attempts have been made to extrude coating materials in a common extrusion zone where ribbons of two different coating materials are extruded side-by-side and in contact with each other. Examples of this type of technique are illustrated in U.S. Pat. Nos. 3,807,918 and 3,920,862. However, difficulties have been encountered with these techniques, particularly when materials of different viscosities are employed. For example, when two different materials of significantly different viscosities are introduced into a common chamber and thereafter extruded through a common extrusion zone defined by upper and lower lands of an extrusion die, the higher viscosity material tends to expand into the area occupied by the lower viscosity material thereby causing enlargement of the width of the stream of higher viscosity material and narrowing of the width of the stream of lower viscosity material. Moreover, difficulty is experienced in achieving uniform edge-to-edge contact between adjacent streams. Attempts to overcome this undesirable characteristic are described in U.S. Pat. No. 3,920,862 wherein one stream of material is introduced on each side of another stream of material to ensure edge contact. Thus the characteristics of common chamber extrusion systems exhibit deficiencies for processes for manufacturing coated articles having precise tolerance requirements.